Modes of the Dark Ages 21 cm field accessible to a lunar radio interferometer.

At redshifts beyond [Formula: see text], the 21 cm line from neutral hydrogen is expected to be essentially the only viable probe of the three-dimensional matter distribution. The lunar far-side is an extremely appealing site for future radio arrays that target this signal, as it is protected from terrestrial radio frequency interference, and has no ionosphere to attenuate and absorb radio emission at low frequencies (tens of MHz and below). We forecast the sensitivity of low-frequency lunar radio arrays to the bispectrum of the 21 cm brightness temperature field, which can in turn be used to probe primordial non-Gaussianity generated by particular early universe models. We account for the loss of particular regions of Fourier space due to instrumental limitations and systematic effects, and predict the sensitivity of different representative array designs to local-type non-Gaussianity in the bispectrum, parametrized by [Formula: see text]. Under the most optimistic assumption of sample variance-limited observations, we find that [Formula: see text] could be achieved for several broad redshift bins at [Formula: see text] if foregrounds can be removed effectively. These values degrade to between [Formula: see text] and 0.7 for [Formula: see text] to [Formula: see text], respectively, when a large foreground wedge region is excluded. This article is part of a discussion meeting issue 'Astronomy from the Moon: the next decades (part 2)'.

Isotropic blackbody cosmic microwave background radiation as evidence for a homogeneous universe.

The question of whether the Universe is spatially homogeneous and isotropic on the largest scales is of fundamental importance to cosmology but has not yet been answered decisively. Surprisingly, neither an isotropic primary cosmic microwave background (CMB) nor combined observations of luminosity distances and galaxy number counts are sufficient to establish such a result. The inclusion of the Sunyaev-Zel'dovich effect in CMB observations, however, dramatically improves this situation. We show that even a solitary observer who sees an isotropic blackbody CMB can conclude that the Universe is homogeneous and isotropic in their causal past when the Sunyaev-Zel'dovich effect is present. Critically, however, the CMB must either be viewed for an extended period of time, or CMB photons that have scattered more than once must be detected. This result provides a theoretical underpinning for testing the cosmological principle with observations of the CMB alone.

Apamin increases post-spike excitability of supraoptic nucleus neurons in anaesthetized morphine-naïve rats and morphine-dependent rats: consequences for morphine withdrawal excitation.

Supraoptic nucleus (SON) oxytocin neurons develop morphine dependence when chronically exposed to this opiate and undergo excitation when morphine is subsequently withdrawn. Morphine withdrawal excitation is evident as an increased action potential (spike) firing rate and is associated with an increased post-spike excitability that is consistent with the expression of an enhanced post-spike afterdepolarization (ADP) during withdrawal. Here, we administered apamin (which inhibits the medium afterhyperpolarization [mAHP] in vitro and unmasks an ADP) into the SON of urethane-anaesthetized rats to determine its effects on oxytocin neurons in vivo. As predicted, intra-SON apamin administration increased the propensity to fire a spike soon (<100 ms) after each spike (post-spike excitability) more in oxytocin neurons recorded from morphine-treated rats than in morphine-naïve rats. However, intra-SON apamin did not alter the overall firing rate of oxytocin neurons recorded from morphine-treated rats or morphine-naïve rats, indicating that an increase in post-spike excitability alone is not sufficient to trigger withdrawal excitation of oxytocin neurons. Nevertheless, bilateral intra-SON apamin infusion increased oxytocin secretion (which depends on firing pattern as well as firing rate) by 90 ± 46% in morphine-dependent rats (P < 0.01 compared to aCSF). Hence, an increase in post-spike excitability does not appear to drive morphine withdrawal-induced increases in oxytocin neuron firing rate, but does contribute to withdrawal-induced hyper-secretion of oxytocin.

The effects of apelin on the electrical activity of hypothalamic magnocellular vasopressin and oxytocin neurons and somatodendritic Peptide release.

Apelin, a novel peptide originally isolated from bovine stomach tissue extracts, is widely but selectively distributed throughout the nervous system. Vasopressin and oxytocin are synthesized in the magnocellular neurons of the hypothalamic supraoptic nucleus (SON) and paraventricular nucleus, which are apelin-rich regions in the central nervous system. We made extracellular electrophysiological recordings from the transpharyngeally exposed SON of urethane-anaesthetized rats to assess the role of apelin in the control of the firing activity of identified magnocellular vasopressin and oxytocin neurons in vivo. Apelin-13 administration onto SON neurons via microdialysis revealed cell-specific responses; apelin-13 increased the firing rates of vasopressin cells but had no effect on the firing rate of oxytocin neurons. A direct excitatory effect of apelin-13 on vasopressin cell activity is also supported by our in vitro studies showing depolarization of membrane potential and increase in action potential firing. To assess the effects of apelin-13 on somatodendritic peptide release, we used in vitro release studies from SON explants in combination with highly sensitive and specific RIA. Apelin-13 decreases basal (by 78%; P < 0.05; n = 6) and potassium-stimulated (by 57%; P < 0.05; n = 6) vasopressin release but had no effect on somatodendritic oxytocin release. Taken together, our data suggest a local autocrine feedback action of apelin on magnocellular vasopressin neurons. Furthermore, these data show a marked dissociation between axonal and dendritic vasopressin release with a decrease in somatodendritic release but an increase in electrical activity at the cell bodies, indicating that release from these two compartments can be regulated wholly independently.

Alpha-melanocyte-stimulating hormone stimulates oxytocin release from the dendrites of hypothalamic neurons while inhibiting oxytocin release from their terminals in the neurohypophysis.

The peptides alpha-melanocyte stimulating hormone (alpha-MSH) and oxytocin, when administered centrally, produce similar behavioral effects. alpha-MSH induces Fos expression in supraoptic oxytocin neurons, and alpha-MSH melanocortin-4 receptors (MC4Rs) are highly expressed in the supraoptic nucleus, suggesting that alpha-MSH and oxytocin actions are not independent. Here we investigated the effects of alpha-MSH on the activity of supraoptic neurons. We confirmed that alpha-MSH induces Fos expression in the supraoptic nucleus when injected centrally and demonstrated that alpha-MSH also stimulates Fos expression in the nucleus when applied locally by retrodialysis. Thus alpha-MSH-induced Fos expression is not associated with electrophysiological excitation of supraoptic neurons because central injection of alpha-MSH or selective MC4 receptor agonists inhibited the electrical activity of oxytocin neurons in the supraoptic nucleus recorded in vivo. Consistent with these observations, oxytocin secretion into the bloodstream decreased after central injection of alpha-MSH. However, MC4R ligands induced substantial release of oxytocin from dendrites in isolated supraoptic nuclei. Because dendritic oxytocin release can be triggered by changes in [Ca2+]i, we measured [Ca2+]i responses in isolated supraoptic neurons and found that MC4R ligands induce a transient [Ca2+]i increase in oxytocin neurons. This response was still observed in low extracellular Ca2+ concentration and probably reflects mobilization of [Ca2+]i from intracellular stores rather than entry via voltage-gated channels. Taken together, these results show for the first time that a peptide, here alpha-MSH, can induce differential regulation of dendritic release and systemic secretion of oxytocin, accompanied by dissociation of Fos expression and electrical activity.

Regulation of neuronal nitric oxide synthase mRNA expression in the rat magnocellular neurosecretory system.

We examined the activation of nNOS mRNA expression within the supraoptic and paraventricular nuclei (SON and PVN) of the hypothalamus. In salt-loaded rats nNOS mRNA expression was significantly increased in both nuclei. In rats given i.p. injections of 1.5 M NaCl (4 ml/kg), a small but significant increase in nNOS mRNA expression in the SON and PVN was found 6 h after injection; no change was detected 2 or 4 h after injection. In rats in which hyponatraemia had been induced experimentally, nNOS mRNA was downregulated in the SON, and expression levels were not increased within 4 h after intense acute osmotic stimuli. Finally, neurons of the SON were antidromically-activated by neural stalk stimulation for 2 h. No increase of nNOS mRNA expression in the SON was observed 2 h after stimulation. Thus, increased electrical activity is not directly coupled to rapidly increased expression of nNOS mRNA, and hence acute increases in nNOS mRNA expression are unlikely to play a role in short-term adaptation of the magnocellular system to osmotic stimulation.

Morphine withdrawal increases intrinsic excitability of oxytocin neurons in morphine-dependent rats.

To determine whether intrinsic mechanisms drive supraoptic nucleus oxytocin neuron excitation during morphine withdrawal, we calculated the probability of action potential (spike) firing with time after each spike for oxytocin neurons in morphine-naive and morphine-dependent rats in vivo and measured changes in intrinsic membrane properties in vitro. The opioid receptor antagonist, naloxone, increased oxytocin neuron post-spike excitability in morphine-dependent rats; this increase was greater for short interspike intervals (<0.1 s). Naloxone had similar, but smaller (P=0.04), effects in oxytocin neurons in morphine-naive rats. The increased post-spike excitability for short interspike intervals was specific to naloxone, because osmotic stimulation increased excitability without potentiating excitability at short interspike intervals. By contrast to oxytocin neurons, neither morphine dependence nor morphine withdrawal increased post-spike excitability in neighbouring vasopressin neurons. To determine whether increased post-spike excitability in oxytocin neurons during morphine withdrawal reflected altered intrinsic membrane properties, we measured the in vitro effects of naloxone on transient outward rectification (TOR) and after-hyperpolarization (AHP), properties mediated by K+ channels and that affect supraoptic nucleus neuron post-spike excitability. Naloxone reduced the TOR and AHP (by 20% and 60%, respectively) in supraoptic nucleus neurons from morphine-dependent, but not morphine-naive, rats. In vivo, spike frequency adaptation (caused by activity-dependent AHP activation) was reduced by naloxone (from 27% to 3%) in vasopressin neurons in morphine-dependent, but not morphine-naive, rats. Thus, multiple K+ channel inhibition increases post-spike excitability for short interspike intervals, contributing to the increased firing of oxytocin neurons during morphine withdrawal.

Regulation of activity-dependent dendritic vasopressin release from rat supraoptic neurones.

Magnocellular neurones of the hypothalamus release vasopressin and oxytocin from their dendrites and soma. Using a combination of electrophysiology, microdialysis, in vitro explants, and radioimmunoassay we assessed the involvement of intracellular Ca(2+) stores in the regulation of dendritic vasopressin release. Thapsigargin and cyclopiazonic acid, which mobilize Ca(2+) from intracellular stores of the endoplasmic reticulum, evoked vasopressin release from dendrites and somata of magnocellular neurones in the supraoptic nucleus. Thapsigargin also produced a dramatic potentiation of dendritic vasopressin release evoked by osmotic or high potassium stimulation. This effect is long lasting, time dependent, and specific to thapsigargin as caffeine and ryanodine had no effect. Furthermore, antidromic activation of electrical activity in the cell bodies released vasopressin from dendrites only after thapsigargin pretreatment. Thus, exposure to Ca(2+) mobilizers such as thapsigargin or cyclopiazonic acid primes the releasable pool of vasopressin in the dendrites, so that release can subsequently be evoked by electrical and depolarization-dependent activation. Vasopressin itself is effective in inducing dendritic vasopressin release, but it is ineffective in producing priming.

Phasic bursts in rat magnocellular neurosecretory cells are not intrinsically regenerative in vivo.

Vasopressinergic hypothalamic magnocellular neurosecretory cells fire in phasic bursts. Burst initiation involves summation of postsynaptic potentials to generate action potentials. Action potentials are each followed by a nonsynaptic depolarizing after-potential that summates temporally to generate a plateau potential and so sustain activity throughout the burst. It is unknown whether this plateau potential exceeds spike threshold in vivo to cause intrinsic regenerative firing or simply approaches threshold to increase the probability that excitatory postsynaptic potentials will trigger further action potentials. Here we show that pharmacological blockade of ionotropic glutamatergic transmission by microdialysis application of kynurenic acid into the supraoptic nucleus of anaesthetized rats prevents spontaneous bursts and bursts (after-discharge) evoked by short trains of antidromically stimulated action potentials in magnocellular neurosecretory cells. Even during prolonged depolarization induced by 1 m NaCl infusion, kynurenic acid microdialysis application still blocked after-discharge. The ability of kynurenic acid to block after-discharge during osmotic stimulation was not caused by an unmasking of inhibitory postsynaptic potentials as kynurenic acid was equally effective in the presence of the ionotropic gamma-aminobutyric acid receptor antagonist, bicuculline, nor did it result from inhibition of plateau potential amplitude as this was unaffected by kynurenic acid and bicuculline in vitro, as was after-discharge evoked in vitro. We conclude that phasic bursts are nonregenerative in vivo but rather require continued excitatory synaptic input activity superimposed upon a subthreshold plateau potential to sustain burst activity.

Intracellular calcium stores regulate activity-dependent neuropeptide release from dendrites.

Information in neurons flows from synapses, through the dendrites and cell body (soma), and, finally, along the axon as spikes of electrical activity that will ultimately release neurotransmitters from the nerve terminals. However, the dendrites of many neurons also have a secretory role, transmitting information back to afferent nerve terminals. In some central nervous system neurons, spikes that originate at the soma can travel along dendrites as well as axons, and may thus elicit secretion from both compartments. Here, we show that in hypothalamic oxytocin neurons, agents that mobilize intracellular Ca(2+) induce oxytocin release from dendrites without increasing the electrical activity of the cell body, and without inducing secretion from the nerve terminals. Conversely, electrical activity in the cell bodies can cause the secretion of oxytocin from nerve terminals with little or no release from the dendrites. Finally, mobilization of intracellular Ca(2+) can also prime the releasable pool of oxytocin in the dendrites. This priming action makes dendritic oxytocin available for release in response to subsequent spike activity. Priming persists for a prolonged period, changing the nature of interactions between oxytocin neurons and their neighbours.

Thapsigargin-induced mobilization of dendritic dense-cored vesicles in rat supraoptic neurons.

Dense-cored vesicles (DCVs) containing oxytocin or vasopressin are secreted from both the nerve terminals in the posterior pituitary and dendrites in the hypothalamus of magnocellular supraoptic neurons. Dendritic secretion can be enhanced (primed) by pretreatment with either thapsigargin or oxytocin for subsequent activity-dependent release. Here, we determined whether priming involves a translocation of DCV closer to the dendritic membrane. To reduce total vesicle content, rats were salt-loaded for 24 h before application of thapsigargin or vehicle onto the ventrally exposed surface of the supraoptic nucleus in vivo. Tissues were then prepared for quantitative electron microscopic analysis of the total incidence of DCVs within supraoptic dendritic cross-sections, and of the incidence and distance (within a 500-nm margin) of each DCV to the dendritic plasma membrane. Salt loading per se did not alter the frequency distribution or average proportion of DCVs found in the 500-nm margin but significantly decreased the average incidence of DCVs per dendrite by 30% (P < 0.05). However, thapsigargin treatment resulted in a significant increase in the total incidence of DCVs within the 500-nm margins and a higher incidence of DCVs within the first 200 nm of the plasma membrane (P < 0.05), indicating that the thapsigargin-induced priming involves a relocation of DCVs closer to sites of secretion.

The role of nitric oxide in morphine dependence and withdrawal excitation of rat oxytocin neurons.

Magnocellular oxytocin neurons develop morphine dependence after intracerebroventricular infusion for 5 days as revealed by their profound excitation following naloxone-induced withdrawal. Oxytocin neurons strongly express nitric oxide synthase (NOS) and nitric oxide (NO) inhibits their activity. This study investigated whether excitation of oxytocin neurons during morphine withdrawal involves reduced activity of NOS and NO. Neuron activity was measured in urethane-anaesthetized rats with blood sampling for oxytocin radioimmunoassay and extracellular single unit firing rate recording of supraoptic nucleus oxytocin neurons. To compare morphine-dependent and -naive rats oxytocin secretion was measured during stimulation by intravenous hypertonic saline infusion. Prior treatment with Nomega-nitro-l-arginine methyl ester, a NOS inhibitor, facilitated osmotically stimulated oxytocin secretion in both morphine-dependent and -naive rats. The facilitation was not different between these groups when corrected for the slower responses observed in morphine-dependent rats. Treatment of morphine-dependent rats with Nomega-nitro-l-arginine methyl ester also enhanced oxytocin secretion during naloxone-precipitated withdrawal. Oxytocin neurons excited by withdrawal were recorded during microdialysis application to the supraoptic nucleus of the NO donor sodium nitroprusside alone and in combination with the GABAA antagonist bicuculline. Sodium nitroprusside inhibited oxytocin neurons during naloxone-precipitated morphine withdrawal and, while bicuculline alone increased firing rate, it did not reduce the inhibition by sodium nitroprusside, in contrast with previous findings in naive rats. Together, these findings indicate that NO restraint of oxytocin secretion is not curtailed during morphine dependence and remains a potent inhibitor of withdrawal excitation despite reduced effectiveness on GABA innervation of the supraoptic nucleus. Hence there is no evidence that changes in NO regulation underlie excitation of oxytocin neurons during opiate withdrawal in morphine dependence.

GABA selectively controls the secretory activity of oxytocin neurons in the rat supraoptic nucleus.

Recently we reported that a single social defeat experience triggers the release of oxytocin (OXT) from somata and dendrites, but not axon terminals, of neurons of the hypothalamic-neurohypophysial system. To further investigate the regulatory mechanisms underlying this dissociated release, we exposed male Wistar rats to a 30-min social defeat and monitored release of the inhibitory amino acids gamma amino butyric acid (GABA) and taurine within the hypothalamic supraoptic nucleus (SON) using microdialysis. Social defeat caused a significant increase in the release of both GABA and taurine within the SON (up to 480%; P < 0.01 vs. prestress release). To reveal the physiological significance of centrally released GABA, the specific GABAA-receptor antagonist bicuculline (0.02 mm) was administered into the SON via retrodialysis. This approach caused a significant increase in the release of OXT both within the SON and into the blood under basal conditions and during stress (up to 300 and 200%, respectively; P < 0.05 vs. basal values), without affecting plasma vasopressin. Electrophysiological studies confirmed the selective action of bicuculline on the firing activity of OXT neurons in the SON. Taken together, our data demonstrate that GABA is released within the SON during emotional stress to act as a selective inhibitor of both central and peripheral OXT secretion.